Surgical tourniquet systems are widely used to facilitate surgical procedures on portions of arms or legs of patients by occluding blood flow into those limb portions for periods ranging from a few minutes to several hours. During operations performed under intravenous regional anesthesia, the surgical tourniquet system serves an additional role of preventing local anesthetic agent introduced into the veins of the operative limb distal to the occlusion from flowing out of the limb into the circulatory system.
A surgical tourniquet system at present typically includes a cuff with an inflatable bladder for encircling the limb and for applying pressure to the encircled limb, pressure regulating means intended for maintaining the pressure within the cuff bladder near a desired pressure, and means for setting the desired pressure, either at a constant level or varying in response to a changing parameter such as systolic, mean or diastolic blood pressure.
Operative and post-operative complications associated with the use of all tourniquet systems described in the prior art are related almost entirely to the pressure applied by the cuff to the limb and hence to the underlying vessels and tissues. As described in the cited prior art, application of insufficient pressure to the limb by the cuff will permit blood to flow into the field of surgical dissection, thereby complicating the procedure and potentially extending the duration of the operation significantly. This is a particularly hazardous situation for operations performed under intravenous regional anesthesia (IVRA). Application of insufficient pressure to the limb by the cuff during administration of IVRA may allow anesthetic agent injected into the veins of the limb to escape into the circulatory system in high concentration, which can cause serious adverse reactions such as cardiovascular collapse, respiratory depression, epileptic seizures or even death.
The results of tests performed on animals, cadavers and to some extent humans, as described in the cited prior art, clearly indicate that high pressures and high pressure gradients applied to the limb by the cuff are associated with higher probabilities of underlying nerve injury, whereas lower applied pressures and lower applied pressure gradients are associated with lower probabilities of nerve injury. These recognized hazards and experimental findings would appear to have important implications for surgical tourniquet systems, but surprisingly no tourniquet system known to the applicant in the prior art regulates the gas pressure within the inflatable bladder of the cuff and simultaneously estimates the pressure actually applied by the cuff to the limb near one or more predetermined locations relative to the cuff.
Recently, research and clinical investigations were completed using a tourniquet cuff incorporating a novel biomedical pressure transducer disclosed by McEwen in U.S. Pat. No. 4,869,265 issued Sep. 26, 1989. The clinical investigations performed using the novel biomedical transducer have revealed many significant discrepancies between the pressure maintained in the inflatable bladder of the pneumatic tourniquet cuff and the pressure actually applied by the cuff to the encircled limb. Normal variations in technique used to apply the cuff to the limb were found to vary the maximum pressure actually applied to the limb by 50 percent or more in comparison to the bladder pressure of the cuff, which was regulated within +/-1% of a preassigned reference pressure by the tourniquet system. In light of the hazards described previously, such a variation is clearly undesirable and in many clinical situations, such as in the use of intravenous regional anesthesia, such a discrepancy can be life-threatening.
Further tests with a tourniquet cuff including the above-referenced biomedical pressure transducer have revealed significant pressure variations in the pressure applied to a limb in a circumferential direction, particularly in the region where the cuff overlaps itself as it encircles the limb. As described above, these pressure variations can result in serious surgical complications.
No tourniquet system in the prior art known to the applicant accurately estimates the level of pressure actually applied to the limb by an inflatable cuff of such a system near a predetermined location, and also employs that estimate of the applied pressure to control the pressure regulator of the tourniquet system so that the pressure applied by the cuff to the limb is maintained near a predetermined reference pressure. Moreover, the applicant is unaware of any surgical tourniquet system known in the prior art which compares the difference between the level of pressure actually applied to the limb by the cuff and the pressure in the inflated bladder of the cuff for the purpose of generating an alarm if a significant difference exists. The applicant is also unaware of any surgical tourniquet system known in the prior art which produces an indication of the applied pressure to the clinical user of the tourniquet system. In addition to mitigating the aforementioned operative and post-operative complications, features such as these would be useful for detecting and indicating cuff misapplication, calibration error in one or more pressure sensors, or pressure sensor failure.
An object of the present invention is to provide a tourniquet system comprising an inflatable occlusive cuff for encircling and applying an occlusive pressure to a limb of a subject, means for controlling the pressure to which the cuff is inflated, means for sensing the pressure applied by the inflated cuff to the limb in a direction normal to the inner surface of the cuff at a predetermined location, and including pressure comparison means for producing an alarm if the difference between the pressure to which the cuff of the tourniquet system is inflated and the pressure applied by the inflated cuff to the limb exceeds a preassigned safety limit.
Another object of the present invention is to provide a tourniquet system comprising an inflatable occlusive cuff for encircling and applying an occlusive pressure to a limb of a subject, means for sensing the pressure applied by the inflated cuff to the limb in a direction normal to the inner surface of the cuff at a predetermined location, and means responsive to the pressure applied by the inflated cuff to the limb for regulating the pressure to which the cuff is inflated so that the pressure applied by the inflated cuff is maintained near a predetermined reference pressure. A related object is to include with this tourniquet system pressure comparison means for producing an alarm if the difference between the pressure to which the cuff is inflated and the pressure applied by the inflated cuff to the limb exceeds a preassigned safety limit.
A further object of the present invention is to provide a tourniquet system comprising an inflatable occlusive cuff for encircling and applying a plurality of pressures to occlude a limb of a subject, means for controlling the pressure to which the cuff is inflated, means for sensing the plurality of pressures applied by the inflated cuff to the limb in a direction normal to the inner surface of the cuff at a plurality of predetermined locations, and including pressure comparison means for producing an alarm if the difference between the pressure to which the cuff of the tourniquet system is inflated and the pressure applied by the inflated cuff to the limb at one selected location of the plurality of predetermined locations exceeds a preassigned safety limit. A related object is to include with this tourniquet system calibration checking means for producing a calibration error signal if the difference between the pressures applied by the inflated cuff at two selected locations of the plurality of predetermined locations exceeds a preassigned difference limit.
The applicant is aware of the following United States patents which are more or less relevant to the subject matter of the applicant's invention.
______________________________________ 4,869,265 9/1989 McEwen 128/774 128/327 4,770,175 9/1988 McEwen 128/327 4,605,010 8/1986 McEwen 128/686 4,479,494 10/1984 McEwen 128/327 128/682 4,469,099 9/1984 McEwen 128/327 128/682 ______________________________________
The applicant is also aware of the following published references which are more or less relevant to the subject matter of the applicant's invention.
J. A. McEwen and R. W. McGraw, "An adaptive tourniquet for improved safety in surgery." IEEE Transactions in Biomedical Engineering, Vol. BME-29, February 1982, pp. 122-128. PA0 J. A. McEwen and G. F. Auchinleck, "Advances in surgical tourniquets." J. Assn. Operating Room Nurses, Vol. 36, 1982, pp. 889-896. PA0 J. A. Shaw and D. G. Murray, "The relationship between tourniquet pressure and underlying soft-tissue pressure in the thigh." The Journal of Bone and Joint Surgery, Vol. 64-A, 1982, pp. 1148-1152. PA0 A. C. McLaren and C. H. Rorabeck, "The pressure distribution under tourniquets." The Journal of Bone and Joint Surgery, Vol. 67-A, 1985, pp. 433-438. PA0 R. J. Newman and A. Muirhead, "A safe and effective low pressure tourniquet." Journal of Bone and Joint Surgery, Vol. 68-B, 1986, pp. 625-628. PA0 J. A. Shaw, W. W. Demuth, and A. W. Gillespy, "Guidelines for the use of digital tourniquets based on physiological pressure measurements." The Journal of Bone and Joint Surgery, Vol. 67-A, 1985, pp. 1086-1090. PA0 S. E. Grice et al., "Intravenous regional anesthesia: Evaluation and prevention of leakage under the tourniquet." Anesthesiology, Vol. 65, pp. 316-320, 1986.